1. Field of the Invention
The present invention relates to a method and an apparatus for cleaning out a wellbore prior to running casing. More particularly, the present invention is adapted for cleaning out caked cuttings which have gravitationally settled out of the mud returns in a highly-inclined wellbore.
2. Summary
A recent trend in the area of oil well drilling is to drill high angle, extended reach wellbores in order to enhance the effectiveness of the well in draining the reservoir. These horizontal and nearly horizontal wellbores create special problems with respect to handling of the mud returns.
In drilling a wellbore, particularly a wellbore with a high angle of inclination from the vertical, a need arises to clean out the wellbore prior to running casing. This is true in both cased and uncased wellbores. With conventional circulation techniques (i.e., down the drill string, out the cutter bit, back up the annulus) in these high angle bores, cuttings tend to gravitationally settle out of the mud returns, and cake on the bottom of either the newly cut portion of the bore or of a formerly cased section. When the next section of casing is run, the caked cuttings will be pushed along by the leading edge of the casing producing a wall of cuttings that will eventually jam, preventing the casing from being run to its intended depth. Further, settled cuttings can cause problems with running the drill string and/or logging tools into and out of the hole.
The present invention comprises a method and apparatus for cleaning out a cased or uncased wellbore (particularly a high angle wellbore) prior to running a(nother) string of casing or logging tool into the hole. A stiff-bristled scouring brush is run in the hole on the leading end of a drill string. Means is provided for rotating the brush as it is advanced through the section of the wellbore to be cleaned to enhance dislodgement of the caked on cuttings, mud, etc. Drilling fluid is reverse-circulated down the casing to entrain the dislodged material and sweep it uphole in the high velocity stream inside the drill string. A dolly helps maintain the cleaning apparatus centered in the wellbore. A resilient spring mounting may be provided for the casters of the dolly to enable them to better accomodate diameter variations of an uncased well.
Various characteristics, advantages and features of the method and apparatus will become apparent after a reading of the following description.